Touch pads are well known, especially for portable devices such as laptop computers and mobile telephones. A touch pad is an input device, and includes a sensor and associated circuitry. When a user moves a stylus or finger to touch a part of the touch pad, that contact affects the sensor and is detected by the circuitry. There are various mechanisms for detecting the point of contact on the touch pad.
One such mechanism is shown in FIG. 1. This sensor 10 utilises a rectangular conductive sheet 11 with an electrical connection 12 at each corner thereof. The conductive sheet 11 is made of homogenous graphite paper.
The conductive sheet 11 is resistive. When a stylus or finger contacts the conductive sheet 11, the resistance at the contact point is changed, so the capacitance (and thus impedance) between two different ones of the electrical connections 12 is changed. To determine the location on the conductive sheet at which the stylus or finger is placed, some measurements are made. There are a number of options for making measurements. In one technique, two adjacent electrical connections 12 are shorted. Then, an AC (alternating current) charge pulse is applied to one of the electrical connections 12 and signal measurements are made at the other two relevant electrical connections (there is no need to measure both of the electrical connections 12 which are shorted together). Afterwards, the shorted electrical connections 12 are disconnected and the opposite two electrical connections 12 are shorted. The measurement process is then repeated with this particular arrangement. From the measurements, one coordinate of the stylus or finger position can be calculated. Next, two other adjacent electrical connections 12 are shorted, and the measurement process repeated. After this, the opposite electrical connections 12 are shorted and the measurement process is repeated.
Signal measurements are used to infer charge distribution, which is dependent on the applied charge pulses and the finger location. Thus, finger location can be determined from the signal measurements. The signal measurements can be made either using a certain charge level as a trigger calculating the number of triggered events with a given time-interval, or by fixing a time interval and determining the charge level at the end of the time period to a reference capacitance value.
In another technique, AC pulses are applied at a corner and measurements are made at the other corners.
In a further technique, AC pulses are applied at all four corners simultaneously.
The application of AC pulses, the current measurement and the position calculation functions are performed by an integrated circuit (IC) such as one of the products vended by Quantum Research Group of Southampton, UK. The position calculations depend on what AC pulse application technique is used.
Sensors such as that shown in FIG. 1 tend to suffer from so-called pincushion error. Pincushion error is greatest at the midpoints of the edges of the sensor 10, and is lowest at the midpoint of the sensor and at the corners of the sensor. Pincushion error results because of loss of charge to corners opposite the edge that the finger is near. Thee pincushion error is greatest at the midpoints of edges because the relative impedance is greatest there; the distance (and thus impedance) between the finger location and the closest corner is not much less than the distance (impedance) between the finger location and an opposite corner.
Since pincushion error produces location errors, it can be inconvenient for a user since the input device may register an input different to that intended. This is particularly inconvenient with touch screen devices, although it is inconvenient also for touch pads and the like.
Pincushion error can be corrected in software. However, software pincushion error correction does not enhance measurement resolution near the corners of the sensor 10, and position measurement accuracy thus is sub-optimal. Furthermore, relatively complex algorithms are needed to make pincushion error corrections, and this provides a burden on processing resources and, significantly, increases power consumption.
FIG. 2 illustrates a prior art sensor 14 which addresses the problem of pincushion distortion in hardware. The sensor 14 is a homogenised carbon sheet, as with the FIG. 1 sensor. The sheet 15 includes plural elongate apertures therein. Two such apertures are labelled at 16 and 17. The apertures 16, 17 extend parallel to one another and parallel to two sides of the rectangle that constitutes the sheet 15. The apertures 16, 17 extend for most of the length of the sheet 15, but leave material such that each corner is connected directly to each adjacent corner by a respective straight track of carbon sheet. The apertures 16, 17 define tracks which run parallel to two sides of the sheet 15. Three such tracks are labelled at 18, 19 and 20. The length of the conductive path between a mid-point of the uppermost side and an adjacent corner (and thus the impedance therebetween) is significantly shorter than the length of the conductive path between the mid-point of the uppermost side and an opposite corner. Thus, charge either takes a longer path to travel to the opposite corner, or a capacitive connection is formed between adjacent stripes. These factors result in the sensor 14 suffering from less pincushion error with a finger located near the uppermost edge. The same effect applies with a finger located at the bottommost edge. Pincushion error is however only slightly reduced with finger locations near the side edges of the sensor 14.
The invention was made in this context.